I believe they've already said that it will replace Gecko on Android and in Firefox OS (this year). It's 2x faster than Gecko on a single core and 3x faster on four cores.

If it's really that good (and so much safer, as Firefox tends to be the browser with the most vulnerabilities) then it would be crazy for them not to build it into a full browser eventually. Microsoft is doing it, so they should do it as well.

Maybe with a new browser they can make Electrolysis be much more optimized as well. I think the main reason it takes them so long to get multi-process to work is because they have to optimize around Firefox's quirks, and I'm worried it still won't end up as safe or as quick as Chrome's multi-process system, which was built for Chrome from day one. In a new browser it might work much better.